Ring artifacts also known as simply rings are a persistent problem in almost all commercially sold CT-scanners when the source and detector rotate as a unit around the object under examination or the source and detector are alternatively fixed and the object rotates on a turntable. This is called rotate-rotate geometry, also known as third generation geometry. The ring artifacts are associated with various medical scanners such as diagnostic head and body scanners based on a torus-design as well as interventional scanners and on-board scanners on radiotherapy machines based on C-arm design. Furthermore, the ring artifacts affect many industrial CT-scanners.
It appears that rings arise in CT-images from nonlinearities among the different sensors in the detector. For example, consider all detector elements of the detector to be perfectly balanced with the same response for the same input signal except for one element. Because of the nature of the rotate-rotate geometry, any detector element including the unbalanced detector element records the transmission ray paths tangent to a circle of fixed radius. The radius of the artifact is determined by how far the unbalance element is from the column of elements that nearly coincide with the projection of the rotation axis of the scanner. Consequently, during the backprojection step in the reconstruction process, the anomalies along these tangent ray paths constructively build up to form a ring or arc in the image.
Rather than eliminating the nonlinearities in the detector elements, prior art has attempted to remove the ring artifacts in the data or images. Prior art has attempted three basic approaches to remove ring artifacts including calibration, filtering out ring-causing components in the data (sinogram) domain and filtering out the rings in the image domain. Although many prior art CT-systems appear to use all three approaches, the details of ring artifact removal algorithms are closely held trade secrets amongst the commercial manufacturers. In any case, in spite of the above prior art techniques, a practical solution is still desired for a method and a system for substantially reducing ring artifacts.